0:13Skip to 0 minutes and 13 secondsSPEAKER: By the end of this presentation, you should be able to describe the World Health Organization's classification of visual impairment. This classification is based on Section H54 of the ICD-10, the International Classification of Diseases. ICD-10 is the standard diagnostic tool for epidemiology, health management, and clinical purposes. It was endorsed by the 43rd World Health Assembly in May 1990, and it came into use in WHO member states in 1994. Visual acuity is an objective, quantitative measure of visual function, and it is one of the first clinical examinations in eye care. It can be carried out in various settings and conditions using a range of standardised charts, distances, and illumination.

0:59Skip to 0 minutes and 59 secondsVarious charts are used across the world. The most commonly used are the Snellen chart, Landolt C, E chart, and the LogMAR chart. The characters on the charts, known as optotypes, are arranged in a standardised way to test distance vision. The most recent arrangements is a LogMAR chart. This is based on proportional spacing, geometric progression, Sans-serif letters, and five letters to each line. Visual acuity measurements made using one chart can be converted to other chart measurements. For example, a visually acuity of 6/60 on a Snellen chart is 0.1 on a Decimal scale and 1.0 on a LogMAR scale. In order to understand the epidemiology of visual impairment, we need to use an agreed definition of visual acuity.

1:49Skip to 1 minute and 49 secondsThis will allow us to compare studies and also to measure impact. The World Health Organization has agreed on the use of four levels of visual function, as described in the ICD-10. These four levels are mild or no visual impairment, moderate visual impairment, severe visual impairment, and blindness. Each of these four levels has a clear, quantitative measure. Moderate visual impairment and severe visual impairment are grouped together under the term low vision. And when low vision is taken together with blindness, it represents all visual impairment.

2:27Skip to 2 minutes and 27 secondsMeasurement of visual acuity is based on distance vision, or visual field assessment. A Snellen or equivalent chart is used at a distance of six metres or 20 feet in a well-illuminated environment. The patient reads across and down the chart as far as they can. Each eye is tested independently and recorded. WHO categories are based on the visual acuity of the better eye in presenting vision. Pinhole is not used when using this method. Presenting visual acuity provides an understanding of refractive need, and is important, particularly for epidemiological surveys and for planning. The WHO's four levels of visual acuity are categorised and measured as follows. Category 0 is mild or no visual impairment.

3:13Skip to 3 minutes and 13 secondsIn this category, vision is equal to or better than 6/18 using Snellen chart measurements. Category 1 is moderate vision impairment. In this category, vision is worse than 6/18 but equal to or better than 6/60. Category 2 is severe visual impairment. In this category, vision is worse than 6/60, but equal to or better than 3/60. The final category is Category 3, blindness. In this category, vision is worse than 3/60.

3:45Skip to 3 minutes and 45 secondsIt's important to remember that visual status categories are based on visual acuity as measured in the better eye. Blindness is also used if the visual field is less than 10 degrees from the point of fixation.

3:59Skip to 3 minutes and 59 secondsIn summary, visual acuity management is an objective measure of visual function, and it is used to categorise and define vision impairment in epidemiological research, public health planning, and screening programmes. Finally, by using the ICD-10 standard definition and categorisation of visual impairment, we are able to compare data and understand the epidemiology of visual impairment.

Defining visual impairment

In order to understand the epidemiology of visual impairment we need to use an agreed definition of visual acuity. This will allow us to compare studies from different locations and also measure impact. In this step we look at the World Health Organization’s (WHO) classification of visual impairment, which will be used throughout the course. How can this classification be applied to your own work?

Visual acuity conversion chart

Throughout the course we use the Snellen (metre) measurement of visual acuity. In other contexts Decimal or LogMAR measurement systems are preferred. The chart shown below allows you to convert from one system to another.